An antilock brake system widely used in automobiles for efficient and safety braking without locking of wheels detects the rotation speeds of wheels by a rotation speed detector (wheel speed sensor), calculates the acceleration and the deceleration and estimates the vehicle speed and the slip ratio by a controller, and drives an actuator to control the brake fluid pressure based on the calculation and estimation results, for example.
Bearing devices including such a rotation speed detector in a roll bearing for supporting automobile wheels (hub bearing) are also widely used. Such a bearing device may be structured such that a magnetic encoder having N and S poles alternately arranged at constant intervals in the circumferential direction is fitted to the inner ring at one axial end portion of a bearing, and a protective cover is press-fitted into the outer ring at one axial end portion of the bearing (for example, refer to Patent Document 1).
The protective cover of the invention described in Patent Document 1 is formed from a synthetic resin in a covered cylindrical shape and is provided with a sensor holder part that holds a magnetic sensor for detecting the rotation of the magnetic encoder (refer to Patent Document 1).
While the protective cover of the invention described in Patent Document 1 is used, the magnetic sensor is opposed to the magnetic encoder with a division wall integrally formed in the protective cover therebetween.
Accordingly, using the protective cover eliminates the need to incorporate a rubber seal member such as an O ring between the wall surface in which a sensor attachment hole is formed and the magnetic sensor as in the protective cover with the sensor attachment hole penetrating in the thickness direction (see FIG. 3(b) describing the related art section of Patent Document 1).
In addition, by providing the protective cover with the sensor holder part, the one axial end portion of the bearing is sealed to keep pebbles and muddy water off the magnetic encoder. This prevents breakage of the magnetic encoder and eliminates the need for the seal member on the outer side of the magnetic encoder. Accordingly, sliding resistance decreases to reduce the rotation torque of the bearing device and obviate the trouble of adjusting an air gap between the magnetic encoder and the magnetic sensor.